The present invention relates to a coupling for attaching tubes, pipes, conduits and other round sections to other such items and/or other systems.
Initially this invention has been developed within the instrumentation tube fittings industry. However, the invention is capable of being utilised within many other applications outside of this environment. Other applications ma include shipbuilding, aerospace, automotive, pipe connections, and construction using wire e.g. suspension bridges.
At present a commonly used connector is a ferrule compression fitting comprising one or more ferrules to be threaded over a tube or the like and a tightening device to cause the one or more ferrules to grip the outside of the tube or the like to provide a seal against leakage. GB 928,715 and U.S. Pat. No. 6,131,963 show examples of twin ferrule connectors. WO 2005/019716 discloses a tube fitting for a tube end, the tube fitting comprising a body with an interior bore to receive a tube end and a collar to be threadably coupled onto the body. The collar includes an integral tube gripping ring that co-operates with a camming surface on the body when the collar is threaded onto the body to force the ring to plastically deform into the tube end. WO 2005/019717 discloses a tube fitting for a tube end, the tube fitting comprising a female threaded coupling member and a male threaded coupling member that are threaded together to assemble the fitting onto a tube end. Whilst being threaded together, a ferrule is detached from the female threaded coupling to form a separate ferrule fitting which is plastically deformed against the outer wall of the tube end and forms seals with the male coupling member and the tube end.
The following problems are associated with these designs:
The design relies on the use of controlled material specifications to ensure that the ferrules have enhanced mechanical properties over that of the tube.
The rear ferrule is treated by one of many processes to enhance the surface hardness. This in itself can cause issues as traditional hardening methods such as nitriding induces carbon into the material, which reduces the surfaces corrosion resistance.
Tube selection is critical within the design. The tubing used with these fittings often has to be purchased with special restrictions of some properties such as skin hardness.
Incorrect tube preparation can cause major problems within the assembly of the fitting and this is one aspect that, the manufacturer has no control over.
Longitudinal scratches on the tube can cause leak paths with these fittings. Basic tube handling commonly causes these scratches. These minor surface defects are often manually removed prior to assembly, which adds to the assembly time.
Due to the requirement of enhanced materials it is accepted that compression fittings manufactured from austenitic stainless steel are not suitable for use within certain environments and do not comply with the National Association of Corrosion Engineers (“NACE”) standards.
The method used to assemble as tube into a compression fitting relies on the end user adjusting the fitting until tube grip is achieved such as by using a threaded connection. This is detected by feel and thus every user will have their own opinion of when this is met. Once this point has been achieved the nut of the fitting is tightened by 1¼ turns and not to a specified torque or a dead stop. This is seen by a limitation within certain fields such as the automotive industry.
A variety of methods have been used in the past to overcome these issues.